The present invention relates to a printer, digital copier, facsimile apparatus or similar electrophotographic image forming apparatus including an LED (Light Emitting Diode) array head and a method of producing data for correcting the quantities of light to issue from LEDs arranged on the LED array head.
An LED array head customarily included in an electrophotographic image forming apparatus has a number of LEDs implementing a desired resolution for exposing a photoconductive element. The problem with this kind of head is that a beam profile differs from one LED to another LED. In light of this, it is a common practice to uniform the quantities of light to issue from the LEDs (light quantity correction hereinafter) or to uniform the diameters of dots to be formed on a photoconductive element (diameter correction hereinafter).
For the light quantity correction, a power meter, for example, is used to measure the quantity of light issuing from each LED and increase or decrease a current being fed to the LED. This scheme, however, has the following problems left unsolved. Usually, the variation of the quantity of light to issue form each LED is confined in a range of .+-.2%. Further, the correction is executed on the basis of the total quantity of light including light in the skirt portions of a beam profile. However, because the beam profile differs from one LED to another LED, the above correction causes the diameter of a beam to differ from one LED to another LED. Particularly, when it comes to a halftone image pattern, the LED array head is apt to form vertical stripes in a developed image because dot areas are not uniform in the image. A laser diode is free from this drawback.
On the other hand, the diameter correction is capable of implementing a uniform dot area in a developed image. This scheme, however, renders the quantities of light to issue from the LEDs irregular and thereby varies image density. It is therefore impossible to reduce the vertical stripes with the diameter correction.
Japanese Patent Laid-Open Publication No. 4-288248, for example, proposes to increase the quantity of light of each LED to the upper limit of the dot diameter, thereby uniforming both the size of the dot diameter and the quantity of light. This, however, increases the dot diameter and cannot implement a sufficient resolution.